linux-kernel - Re: [PATCH 01/11] mfd: add the Berlin controller driver

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Message-ID: <20150218084004.GD21937@kwain>
Date:	Wed, 18 Feb 2015 09:40:04 +0100
From:	Antoine Tenart <antoine.tenart@...e-electrons.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Antoine Tenart <antoine.tenart@...e-electrons.com>,
	sebastian.hesselbarth@...il.com, sameo@...ux.intel.com,
	jszhang@...vell.com, zmxu@...vell.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] mfd: add the Berlin controller driver

On Tue, Feb 17, 2015 at 11:54:48AM +0000, Lee Jones wrote:
> On Tue, 17 Feb 2015, Antoine Tenart wrote:
> > On Mon, Feb 16, 2015 at 12:48:08PM +0000, Lee Jones wrote:
> > > On Wed, 11 Feb 2015, Antoine Tenart wrote:
> 
> > > > +static int berlin_ctrl_probe(struct platform_device *pdev)
> > > > +{
> > > > +	struct device *dev = &pdev->dev;
> > > > +	const struct of_device_id *match;
> > > > +	const struct berlin_ctrl_priv *priv;
> > > > +	int ret;
> > > > +
> > > > +	match = of_match_node(berlin_ctrl_of_match, dev->of_node);
> > > > +	if (!match)
> > > > +		return -EINVAL;
> > > > +
> > > > +	priv = match->data;
> > > > +
> > > > +	ret = mfd_add_devices(dev, 0, priv->devs, priv->ndevs, NULL, -1, NULL);
> > > > +	if (ret) {
> > > > +		dev_err(dev, "Failed to add devices: %d\n", ret);
> > > > +		return ret;
> > > > +	}
> > > > +
> > > > +	return 0;
> > > > +}
> > > 
> > > I'm not sure I see the point in this driver.  Why can't you just
> > > register these devices directly from DT?
> > 
> > All these devices share the same bank of registers and we previously
> > used a single node. But with many devices sharing a single node, this is
> > problematic to register all the devices from DT. Using this MFD driver
> > to do it is a proper solution in this case.
> 
> Tell me more.  What are the problems you encountered?

So we had a single node, chip-controller, accessed by multiple
devices -and drivers-. We ended up with:

chip: chip-control@...000 {
	compatible = "marvell,berlin2q-chip-ctrl";
	reg = <0xea0000 0x400>, <0xdd0170 0x10>;
	#clock-cells = <1>;
	#reset-cells = <2>;
	clocks = <&refclk>;
	clock-names = "refclk";

	[pinmux nodes]
};

In addition to being a mess, how can you probe various drivers with this
single node? We had to probe a clock driver in addition to the
pin-controller and reset drivers. We ended up using arch_initcall() in
the reset driver, which was *not* acceptable.

These chip and system controllers are not an IP, but helps not spreading
this bank of registers all over the DT.

The solution to this problem is to introduce an mtd driver which
registers all the sub-devices described by these chip and system
controller nodes.

Antoine

-- 
Antoine Ténart, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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